In a 5- to 6 quart Pan over high heat,bring about 2 quarts water to a boil. Meanwhile,rinse green beans ,trim off ends,and pull off any strings.Cut into 2-to 3inch lengths.Add beans to boiling water and cook until barely tender to bite,3 to 5 minutes.Drain and immerse in ice water until cold,drain well.
Fill same pan with 2 1/2 to 3 quarts water and bring to a boil over high heat .Add orzo and cook until barely tender to bite.8 to 11 minutes.Drain and rinse with cold water,drain well.
Meanwhile,husk corn ,discarding silk;rinse ears.Holding each ear upright in a large,deep bowl,cut off kernels close to the cob.
To make dressing:In a small bowl,whisk together vinegar,olive oil,shallots,mustard,tarragon,and salt and pepper to taste.
In a large,wide bowl,mix orzo with 1/2 cup of dressing.Add more salt and pepper to taste.Spread level. Layer corn kernels,green beans,and tomatoes over pasta.Pour remaining dressing into a container. Cover and chill both salad and dressing until ready to serve.
Shortly before serving, pour three-fourths of the remaining dressing over salad and mix gently to blend. Add more dressing and salt and pepper to taste.